The present invention generally relates to the controlled application of flux. The present invention is particularly related to the application of flux for the selectability of the type of deposition, either spray or droplet(s), the location of the deposition and the selected amount of deposition. Even more particularly, the present invention is related to the dispensing apparatus which can selectively provide either microdeposits or finely controlled atomization in a single device while being able to vary deposition properties rapidly during the application process. In addition, delivery of the flux to the dispensing apparatus is precisely coordinated with the dispense parameters.
This invention pertains to a method and apparatus for controlling the dispensing of fluid materials, and more particularly to a method and apparatus for controlling the dispensing of flux on an electronic assembly such as a printed circuit board, or on components used thereon.
The invention is particularly applicable to dispensing flux with a solids content less than 15% and a viscosity of less than 100 cps, and more specifically, a viscosity less than 50 cps. However, it will be appreciated that the invention has broader application and may be advantageously employed with other types of fluxes. In the assembly of a printed circuit board (PCB) the soldering process is one of the most critical steps. Soldering has traditionally been done with a process known as wave soldering. Known structures and methods for applying flux to a printed circuit board prior to the wave soldering step include liquid wave, foaming, brushing or spraying as described, for example in U.S. Pat. No. 5,328,085.
The disadvantages of each of these processes are also described in U.S. Pat. No. 5,328,085. While the prior art has reduced many of the problems of applying flux there are new requirements for more selective flux application, greater control of deposition and increased through-hole penetration of the flux. In addition, there is now a desire to flux only selected areas with a thin application of flux. These needs exist in traditional wave soldering equipment and in newer selective soldering applications.
Due to the decreased need for through-hole components, many wave soldering manufacturers are now using pallets with cut-out regions for soldering which are referred to as aperture wave solder pallets. The solder wave still contacts the complete bottom surface of the pallet but only comes in direct contact with the PCB through the open areas in the pallet. The need is for flux application that can either 1) apply flux only to the exposed areas of the board or to 2) apply flux at required quantities in the exposed areas and at a reduced amount over the rest of the bottom surface of the pallet. A small layer of flux on the pallet can extend the life of the pallet so a reduced level of flux on the pallet is desired.
In addition, there is a need for better penetration of flux into the holes in the boards while not applying excess flux or leaving a residual amount of flux at the interface of the pallets and PCB.
Also, a new approach to soldering PCBs called selective soldering requires discrete programmable amounts of flux to be applied to the PCB. Selective soldering only solders the through-hole areas and therefore only those selected areas need to have flux applied to them. This approach reduces the amount of flux used and, more importantly, can restrict the heat exposure associated with solder temperatures to only those areas on the board that require wave soldering. Current methods include stationary spray guns positioned under a template which blocks the flux where it is not needed or a programmed spray gun which moves to locations but again applies flux through the open area of a template. Both of these methods require 1) extensive maintenance due to the excess flux produced, 2) extensive tooling for each board, 3) additional setup time for changing tooling for each new board lot, and 4) inconsistencies in setting up the multiple spray guns for each discrete location.